This invention generally relates to a Schmitt trigger circuit comprised of complementary MOS transistors and more particularly to a Schmitt trigger circuit using an inverter and at least four transistors connected so as to ensure that its threshold voltages can optionally be set and its hysteresis voltage width can be sufficiently broad.
In a conventional Schmitt trigger circuit of complementary MOS (called CMOS hereinafter) transistors as shown in FIG. 1 comprises P-channel enhancement MOS transistors P.sub.1 to P.sub.4, and N-channel enhancement MOS transistors N.sub.1 and N.sub.2. The transistors P.sub.1 and N.sub.1 are coupled to constitute one inverter, the transistors P.sub.4 and N.sub.2 are coupled to constitute the other inverter, and the transistor P.sub.2 is in diode connection. When an input voltage V.sub.IN changes from "0" to "1" (from GND to V.sub.DD), an output voltage V.sub.OUT is initially "0" and hence the transistor P.sub.3 is rendered on (saturated). Accordingly, the transistor P.sub.1 has then a source voltage which approximates the power source voltage V.sub.DD. Conversely, when the input voltage V.sub.IN changes from "1" to "0" (from V.sub.DD to GND), the output voltage V.sub.OUT is initially "1" until the transistor P.sub.3 is rendered off so that the transistor P.sub.1 has a source voltage which is affected by the transistor P.sub.2 in diode connection and which is decreased from the power source voltage V.sub.DD by a voltage V.sub.TP (threshold voltage of the transistor P.sub.2). Consequently, an inversion voltage V.sub.IL of the output voltage V.sub.OUT upon the change of the input voltage V.sub.IN from "1" to "0" decreases below an inversion voltage V.sub.IH upon the change from "0" to "1", providing a hysteresis characteristic in an input/output voltage characteristic as shown in FIG. 2.
With the conventional circuit of FIG. 1, however, the degree of freedom for setting the inversion voltages V.sub.IL and V.sub.IH is restricted, particularly, the hysteresis voltage (V.sub.IH =V.sub.IL) is limited to about 0.2 volts and the application of this circuit is limited to particular fields.